The present invention relates to an electron gun for a cathode-ray tube, and more specifically to an electron lens of an electron gun assembly for focusing at least one electron beam, preferably two or more electron beams.
Conventionally, a cathode-ray tube includes at least one electron gun. The electron gun comprises a beam forming section for producing an electron beam and a main lens section for focusing the electron beam on the target. The spot diameter of the electron beam on the target is a very important factor to determine the performance of the cathode-ray tube. The spot diameter on the target should preferably be minimized, depending on the performance of the electron gun. Improvement of the performance of the main lens section is an effective measure for improving the performance of the electron gun as a whole.
The main lens section is chiefly composed of an electrostatic electron lens. In the electron lens region, electrodes, each having an aperture, are coaxially arranged so as to be applied with predetermined voltages. There may be several types of such electrostatic electron lenses which vary according to the variety of voltages. For higher performance of the main lens section, however, it is necessary to increase the size of the aperture, thereby increasing the lens aperture in the optical sense, or to lengthen the separation distance of the electrodes to cause a gradual potential change in the region around the electrodes, thereby forming a long-focus lens having a long focal length.
However, such a prior art electron gun for a cathode-ray tube is sealed in a cylindrical glass tube, i.e., the neck portion of a cathode-ray tube. Therefore, the size of the aperture of the electrodes (or the lens diameter) is restricted by the diameter of the cylindrical glass tube. Also, the separation distance of the electrodes is limited so that an electrostatic focusing field formed between the electrodes may not be influenced by any other undesired electric fields in the cylindrical glass tube. In a color picture tube, in particular, if a plurality of electron guns are arranged in line, narrower intervals between the electron guns will make it easier to converge a plurality of electron beams on the same point on the whole surface of a screen. In consideration of deflection, moreover, the narrow intervals between the electron guns improve the economy of electric power. The narrower intervals, however, require a further reduction in the size of the apertures of the electrodes.
In the cathode-ray tube as described above, the lens performance is expected to be improved by the use of a long-focus lens which can produce, without an extension of the separation distance of the electrodes, an effect equivalent to that obtained with use of a longer separation distance. There are proposed several electrostatic electron lenses for such a cathode-ray tube. Among these lenses, for example, there is a "tripotential" and a "single-element bipotential lens" disclosed in U.S. Pat. No. 4,124,810 by Bortfeld et al.
In the single-element bipotential lens disclosed in U.S. Pat. No. 4,124,810, three cylindrical electrodes with the same diameter are arranged along electron beams for low, middle, and high voltages, so that a gradual potential change is produced at the main lens section. Optimum lens performance may be obtained if the length of the middle-voltage electrode is substantially equal to the radius of the electrode aperture. However, using this technology, the lens performance cannot be further improved.
For additional improvement in the lens performance, therefore, the multi-element bipotential lens disclosed in U.S. Pat. No. 3,932,786 has been proposed. In an electron gun using this lens, however, resistors arranged near the individual electrodes are small. Thus, the electron gun of this type is unfit for practical use. Moreover, since the voltages of the electrodes are picked up at narrower intervals from the small resistor, the construction and manufacture of the electron gun are complicated. The small gaps between the electrodes facilitate the flow of leakage current between the electrodes. In consequence, undesired current is produced by the leakage current, beam impact hit on the electrodes and other factors, resulting in a change of electrode potential and lowering the lens performance. These drawbacks make it very hard to put an electron gun of this type into practical use.
To increase the diameter of the electron lens, moreover, electron guns of the following types are conventionally proposed. In an electron gun assembly for a color picture tube disclosed in Japanese Patent Application Disclosure No. 124933/80, three electron lenses are formed overlapping one another. In another electron gun stated in the Proceedings of the Third International Display Research Conference, Japan display 1983, pp. 268 through 271, apertures of electrodes are conical. In an electron gun assembly disclosed in Japanese Patent Application Disclosure No. 103246/82, moreover, projections are formed around three apertures. In these electron guns, the diameter of each electron lens is increased, so that the lens performance is improved in some measure. For further improved lens performance, the separation distance of the electrodes need be increased. This separation distance cannot, however, be increased, since it is influenced by undesired electrostatic fields in the neck.